purchase

purchase, a mechanical device to increase force, whether by means of levers, gears, or blocks, or pulleys rove with a rope or chain. In its maritime meaning it is only the last of these which is known as a purchase: a rope rove through one or more blocks by which the pull exerted on the hauling part of the rope is increased according to the number of sheaves in the blocks over which it passes.

Where two or more blocks are involved in the purchase, it is generally known as a tackle, though there are exceptions to this general rule when two double or two treble blocks are used, these being known as twofold and threefold purchases respectively. The blocks of a tackle are known as the standing block and moving block, the rope rove through them is known as the fall and is divided into three parts known as the standing, running, and hauling parts. The amount by which the pull on the hauling part is multiplied by the sheaves in the blocks is known as the mechanical advantage.

Twofold and threefold purchases are used only for heavy lifting, such as hoisting boats inboard. When rove to advantage, their theoretical mechanical gains are 5 and 7 respectively, when rove to disadvantage, the gains are 4 and 6, though in each case the loss due to friction of the sheaves is considerable since more sheaves are used. Allowing for average friction these gains are reduced to 3.57 and 4.37 when rove to advantage, and 2.26 and 3.75 when rove to disadvantage.

See also gun tackle.